I would like to take to the time to remind players that the theme is used to give the game a little flavor. If you end up being a character from Team Rocket or associated with Team Rocket, that does not necessarily mean you are mafia.

Theory is that because Ash was turned to stone in the 1st Movie by Mew and Mewtwo and was brought back to life by Pokemon tears, he can never age.

And then there is the theory from the first episode when he saw who we later learn to be Ho-Oh. Seeing Ho-Oh will promise you eternal happiness. So him staying ten and traveling the world with Pokemon is probably his eternal happiness.

On a related note I have met old the voice actors of James, Oak, Ash/Ash's mom at the pokecon I went too! However the voice actor of Oak actually got to work on an episode in the newer episodes of pokemon as Oak again.